The present invention relates to the treatment of liquid media containing organic and/or inorganic foulants. More particularly the invention relates to the cleaning of multipore diffusion elements while submerged in liquid media including such foulants.
The aeration of waste liquid media, including for example domestic sewage and industrial waste waters, is an old art. The activated sludge process, which includes aeration of liquors containing domestic sewage, has been in continuous use for about sixty years.
The liquid media treated in such aeration processes very commonly contain organic and/or inorganic foulants such as for example relatively insoluble salts which are responsible for the hardness of the water, and living and non-living organic residues which contribute to the formation of scales and slimes. Upon aeration of these media with submerged aeration devices, there is a tendency for the foulants to progressively foul such devices at the point of release of oxygen-containing gas into the liquid media, closing up or otherwise modifying the openings through which the oxygen-containing gas is released into the media with various undesirable results.
Such fouling can impair the uniformity of gas distribution from aeration devices, especially when such devices are of the area discharge type, such as for example the flat porous ceramic plates which were used to discharge air into sewage liquors in early activated sludge plants. Also, fouling can in certain circumstances increase the pressure differential required to drive oxygen-containing gas through the aeration devices at a given flow rate, thus either reducing the flow of oxygen available, and therefore the oxygen transfer rate of the aeration system, and/or increasing the amount of power consumed in maintaining the desired rate of flow, thus substantially increasing the energy requirements and cost of the process.
Since these fouling phenomena are often progressive in nature they can eventually lead to a complete or near complete disablement of the aerating devices if permitted to continue long enough. However, a long-standing recognition of the intolerable circumstances that can result from failure of a sewage treatment plant has provided considerable encouragement for persons skilled in the art to explore remedial measures.
The fouling problem has been discussed and confronted in various ways with varying degrees of success for many years. Literature references discussing the problem and proposed solutions were available in the 1930s. These and subsequent publications demonstrate the severity of the problem and the elusiveness of any truly satisfactory solution. At a very early stage it was recognized that the removal of diffusion elements from an aeration tank for cleaning was both inconvenient and relatively expensive in view of the labor costs and the loss of use of the facility. Accordingly various attempts were made to develop satisfactory processes for cleaning the aeration devices in place, i.e. without removal from the aeration tanks and, wherever possible, without draining the liquid media from the tanks.
One of the techniques tried was injection of chlorine gas into the aeration system in admixture with air while the aeration system was in operation. A measure of success was obtained in that there was reduction of flow resistance and apparently some prolongation of the life of the elements. However, such techniques were only sporadically successful.
For example, R. B. Jackson reported in his article xe2x80x9cMaintaining Open Diffuser Plates With Chlorine,xe2x80x9d Water Works and Sewerage, September 1942, pages 380-382, that the application of chlorine, whenever required, was effective in maintaining operation for a period of time, following which it again became necessary to drain the aeration tank and clean the decommissioned diffusion elements with liquid cleaners including acids. But Jackson was only one of a number of individuals who experimented with in place cleaning with gaseous cleaning agents in a variety of plants. See for example W. M. Franklin, xe2x80x9cPurging Diffuser Plates With Chlorine,xe2x80x9d Water Works and Sewerage, June 1939, pages 232-233; xe2x80x9cManual of Practice No. 5, xe2x80x9d Federation of Sewage and Industrial Wastes Associations, Champaign, Ill., 1952, pages 60-61; and U.S. Pat. No. 2,686,138 to Klein.
However, despite the early attempts at perfecting this technique, it has not been widely regarded as generally acceptable heretofore for large sewage treatment plants with multipore diffusion elements.
It is of interest to note that sewage treatment plant designers are generally familiar with the tubing-type diffusers for the sewage treatment ponds or lagoons used by small communities. Such systems usually employ rows of small diameter plastic tubing resting on or suspended above the bottom of a lagoon or basin and having small holes or slits formed in the tubing at relatively widely spaced intervals along the length of the tubing. For example, one commercially available type of tubing-type diffuser marketed by Lagoon Aeration Corporation under the trademark LASAIRE is weighted tubing having an inside diameter of approximately one-half inch with a small bore on the order of .012xe2x80x3 in diameter about every four inches along the crown of the tubing. Another commercially available form of tubing type diffuser employs slits instead of bores. Still another type employs rigid plastic tubing having small porous ceramic inserts cemented into the tubing wall instead of the bores or slits previously mentioned. Sanitary engineers are, of course, aware of the successful cleaning of such tubing type diffusers by the addition of a cleaning gas such as hydrogen chloride to the oxygen-containing gas, which mixture is forced through the bores, slits or small ceramic inserts, while the latter are in place submerged in the liquid media, to remove incrustations of organic and/or inorganic foulants.
Notwithstanding the apparent success of in place gas cleaning of tubing-type diffusers and the long-standing knowledge of and early attempts at in place gas cleaning of the multi-pore area release diffusion elements customarily employed in the tank-type aeration facilities generally used by large cities and counties, gas cleaning in place has not been generally adopted for such facilities. Considering the long-standing nature of the fouling problem and the fact that the technology relating to in place gas cleaning of tubing type diffusers has been readily available to sewage plant designers for years, it might seem reasonable to assume that in place gas cleaning would have long ago become the technique of choice for the tank-type aeration facilities equipped with multi-pore diffusion elements. That it has not become a commonly used method bears silent but effective witness to the fact that a practical, economical and dependable technique for in place gas cleaning of multi-pore diffusion elements in tank-type aeration facilities was not obvious to plant designers and operators of ordinary skill in the art.
Further evidence of such non-obviousness is provided by the willingness of facility operators to indulge in such inconvenient, time consuming and expensive measures as removing the unit from service, draining the tank, doing preliminary cleaning of the tank and of the fouled diffusion elements with fire hoses and the like, removing literally tons of elements from the tanks, transporting them to a cleaning facility, subjecting them to acid and/or caustic solution cleaning, drying the elements, refiring them at elevated temperatures, replacing the rather substantial number of elements which are inevitably destroyed by cracking or warping in the refiring process, transporting the elements back to the plant, reinstalling them with removal of damaged gasket material from the holders, installation of new gaskets, retightening and torqueing of the means for holding the diffusion elements in their holders, refilling the tank and returning the facility to operation.
Additional evidence has been provided by a study entitled xe2x80x9cSurvey and Evaluation of Fine Bubble Dome Diffuser Aeration Equipment,xe2x80x9d by Daniel H. Houck and Arthur G. Boon, completed Sep. 1, 1980, in fulfillment of a grant from the Association of Metropolitan Sewerage Agencies and the British Water Research Centre under the partial sponsorship of the U.S. Environmental Protection Agency. While making an in-depth review of the designs, operating procedures, performance and maintenance procedures of U.S. and overseas activated sludge plants equipped with fine bubble diffusers, the investigators surveyed fouling problems and cleaning methods. None of the plants which required periodic cleaning employed in-place gas cleaning. Among the cleaning methods used for ceramic diffusion elements were refiring, acid washing combined with clean water- and steam-cleaning, ultrasonic cleaning, hand brushing and others. The study provided detailed information and observations on the costliness of and limited economic justification for refiring. Nevertheless, it was recommended on grounds of established effectiveness that refiring and/or acid washing be used where possible. But the study also uncovered evidence that acid washing did not adequately clean ceramic diffusion elements fouled with scale, apparently calcium carbonate, and that the diffusion elements should be refired for proper cleaning.
Plainly the need for an in-place gas cleaning apparatus suitable for tank-type plants with the multi-pore diffusion elements has existed for more than forty years. However, the willingness of plant designers, operators and government officials to accept or even promote the above described disruptive, lengthy, troublesome and expensive procedure shows that the solution to the problem is not in fact evident. The present invention is aimed at fulfilling this need.
As suggested in our U.S. Pat. No. 33,177, the holders of diffusion elements used in gas cleaning, as well as their respective retaining means, can take a wide variety of forms, including those which secure the diffusion elements by direct or indirect contact about their entire peripheries, or at spaced points about their peripheries or at other locations. For example, see the center bolt arrangements shown in U.S. Pat. Nos. 4,046,845 to Richard K. Veeder and 3,532,272 to Eric S. Branton. However, as our patent also taught, attachment of the diffusion elements by central or other fasteners which extend through holes in the active diffusion surface produce detrimental effects, the prediction of which would not have been obvious.
In much of the prior art, sealing between element and plenum is accomplished through vertically loaded elastomeric gaskets. The required loading to effect adequate seal of the porous diffusion element may be high, e.g. 50 pounds/lineal inch of seal. Greater strength and rigidity of the diffuser and plenum is required to distribute these forces about the periphery than in the preferred embodiments of this invention wherein continuous peripheral clamping or retaining is employed.
Further, fasteners extending through the element into the plenum typically require holes with clearance. Unless the interiors of the holes are sealed in their entireties, free passage of air is provided in these clearances that promotes excessive flow from the diffusion element in the vicinity of the fastener. Enlarging the sealed area under the lower horizontal surface of the retaining means, to lengthen the path of air from the clearance zone to the diffuser surface does not correct this deficiency, since the reduction in unit flow (flux) in the vicinity of the fastener resulting from the additional sealed area at the surface, similarly reduces the frictional pressure drop in that region, and the problem of non-uniform distribution persists.
The detrimental effects of the xe2x80x9cthrough-holexe2x80x9d type fasteners above described may be overcome by the use of the preferred peripheral clamping or retaining methods employed in our invention.
Possible Theoretical Considerations
In order for a gas to flow through a gas discharge passage of a diffusion element and bubble into a liquid medium, there must be a relatively higher pressure in the gas at the influent end of the passage as compared to the pressure in the liquid at the effluent end of the passage. This difference in pressure may be referred to as a pressure loss.
A substantial portion of the pressure loss is attributable to the force exerted on the gas by liquid surface tension at the location where bubbles are formed. Frictional resistance to the flow of gas in the passage also contributes, but to a lesser degree, to the total pressure loss across the element.
The magnitude of the surface tension component of the pressure loss varies inversely with the effective hydraulic radius of the passage at the location where bubbles are formed. Thus, the surface tension component of pressure loss tends to be larger for smaller passages and vice versa.
The frictional resistance component of pressure loss varies inversely with pore diameter. Thus this component tends to be larger in smaller pores and vice versa.
Pressure loss through a passage is also affected by the rate of flow of gas through the passage. Surface tension forces are not greatly affected by flow rate at typical rates for multi-pore diffusion elements, but the frictional resistance to flow is directly related to the flow rate. Thus an increase or decrease in flow rate through a passage respectively increases or decreases the total pressure loss.
Each diffusion element, when new, for a given gas or gas mixture, under given conditions of flow, temperature, gas viscosity, barometric pressure and humidity, will have a characteristic pressure loss, as well as a characteristic rate of flow at a given pressure loss. The pressure/flow characteristics of diffusion elements are often expressed in terms of dynamic wet pressure. The pressure loss and dynamic wet pressure of an element represent combined effects of different flow through many individual passages throughout the element.
There are significant variations in pressure/flow characteristics among diffusion elements. Individual pores in a given element differ in their pressure/flow characteristics due to differences in the shape, size and hydraulic radii of the passages or pores. Considering the state of the art it is challenging to manufacture a run of hundreds or thousands of diffusion elements in which most or all of the elements exhibit substantially the same pressure/flow characteristics. Moreover, as elements are used in liquid media containing foulants, the foulants can clog the passages or pores at their inlets, interior portions, outlets or any combination thereof in such a manner as to reduce pore diameter and/or effective hydraulic radius, with a consequent effect on the pressure/flow relationships of the element. Thus, all other factors remaining equal or equivalent, fouling tends to reduce the rate of flow at a given pressure or increase the amount of pressure required to maintain a given flow rate.
Without intending to be bound by any theory, the applicants offer the following theoretical explanation which may be indicative of the operation of the invention and the nature of certain advantages which may accrue from its use. When an element is new, it is believed that the flow of gases through the element is preferentially directed through those pores having the lowest combined frictional and surface tension resistance. It appears that other pores with a higher combined frictional and surface tension resistance would remain less active or perhaps inactive, at least temporarily. According to this theory, as the active or most active pores begin to clog through deposits of foulants, flow is reduced. If the system operating pressure is increased in order to maintain flow, progressively more and more of the pores which were originally active or most active continue to clog, and some of the less active pores characterized by higher pressure loss are now able to transmit gas due to the increased pressure. It is visualized that the aforementioned portion of the less active pores will themselves become clogged eventually. If the system pressure again increases, additional less active pores, characterized by an even higher pressure loss, will then become active. It is visualized that progressive clogging and the increasing of pressure to maintain flow can proceed to such an extent that most of the available pores in the diffusion element have been clogged. If in place gas cleaning is performed in a conventional manner at this point in the history of the element, it is theorized that because of the aggravated clogging of the element, the cleaning gas will pass through only a relatively few pores, such as for example, those small pores which were among the last to become active and perhaps some larger pores which were not fully obstructed when cleaning was commenced.
According to this theory, as the cleaning gas passes through the few pores referred to above, and the deposits on and within them are reduced, there is a resultant rapid increase in flow through these passages or pores. Such increase tends to reduce the gas pressure available at the influent surface of the element to drive cleaning gas through other clogged pores or passages which, as a result of clogging, are characterized by a higher resistance to flow than the few passages which have been cleaned. The available pressure may now be so low that it is unable to overcome the effects of frictional resistance and surface tension in many or most of the clogged passages. Unfortunately, if the theory is correct, those passages or pores on which the cleaning will have limited or no beneficial effect are some of those which were the first to clog and which would have had the potential for providing the maximum flow at a given system operating pressure and therefore would have provided the greatest potential for saving power through cleaning.
The foregoing theory, if correct, may explain why diffusion elements were frequently not restored to their original levels of pressure loss by in place gas cleaning. Possibly, such inability may also have resulted from the use of inadequate quantities of cleaning gas. Such failure may in some cases have occurred because there was no provision for individual flow regulating means for each diffusion element or group of diffusion elements. Such lack may have caused the flow of cleaning gas to seek out and pass through those diffusion elements which were among the last to become active in preference to those which had clogged earlier, so that application of the cleaning gas was ineffective in restoring the system to its original condition or flow capabilities.
The adverse effects of delayed frequency of cleaning may be reduced and the efficiency of cleaning according to the invention may be enhanced by a number of techniques and facilities, including but not limited to: division of the diffusion elements in a diffusion system into groups, which groups may be cleaned separately and independently from other groups; providing means for operating the diffusion elements in one of the aforementioned groups, during cleaning, at flow rates and/or pressure differentials higher than the groups in the remainder of the system; provision of flow control means for diffusion elements that will apply relatively higher plenum pressure to elements operating at lower air flow rates and vice versa; temporarily lowering the surface tension at the interfaces of the liquid medium and diffusion elements while the latter are being cleaned; initiating gas cleaning while the diffusion elements are operating at relatively low mean dynamic wet pressure and/or mean bubble release pressure; initiating cleaning prior to the onslaught of load fluctuations creating peak demand for treating gas, such as for example peak seasonal demand conditions of activated sludge plants; and providing means for manipulating liquid levels in the tanks in the system.
The invention is, according to a first aspect, a process for treating a liquid medium comprising the steps of introducing treating gas into a gas distribution network having a submerged portion in said liquid medium in a plant for treating said liquid medium; passing treating gas through a plurality of diffusers having diffusion elements submerged in the medium, wherein foulants in the medium or treating gas may tend to form deposits in the elements or at their surfaces and cause a potential or actual increase in the dynamic wet pressure of the diffusion elements relative to a previous base condition of said elements; and measuring, at a submerged diffusion element in said plant, an operating parameter that is indicative of dynamic wet pressure across the diffusion element.
According to further embodiments of the first aspect, said measuring step is accomplished with precision sufficient for controlling the application of cleaning agent to restrict any potential or actual increase of the dynamic wet pressure across the diffusion elements beyond about 25, or 15 or 7 inches of water gauge above a base condition of said elements, and the application of cleaning agent is controlled in accordance with said measuring step to maintain said restriction on dynamic wet pressure.
There are three additional embodiments, each applicable to the first aspect and each of its above-described further embodiments. The first of these additional embodiments comprises the step of measuring pressure across and/or flow of gas through one or more diffusion elements which are present in the submerged portion of the network, and which constitute only a portion of said plurality of diffusion elements, for controlling the application of cleaning agent. The second additional embodiment comprises the step of measuring pressure across and flow of gas through one or more diffusion elements which are present in the submerged portion of the network, and which constitute only a portion of said plurality of diffusion elements, for controlling the application of cleaning agent. The third additional embodiment comprises the steps of measuring the hydrostatic pressure of said liquid medium at about the elevation of a diffusion element in a submerged portion of the network, measuring the gas pressure within said gas distribution network near an air flow regulating means supplying gas to said diffusion element through a plenum, and measuring gas pressure downstream of the flow regulating means and within the plenum.
According to a second aspect, the invention includes a process of treating a liquid medium comprising the steps of introducing treating gas into a gas distribution network having a submerged portion in said liquid medium in a plant for treating said liquid medium; passing treating gas through a plurality of diffusers comprising diffusion elements submerged in the medium, wherein foulants in the medium or treating gas tend to form deposits in the elements or at their surfaces, causing a potential or actual increase in the dynamic wet pressure of the diffusion elements relative to a previous base condition of said elements; and measuring, at a submerged diffusion element in said plant, changes in operating parameters that indicate dynamic wet pressure changes across the diffusion element.
According to further embodiments of the second aspect, said measuring step is accomplished with precision sufficient for controlling the application of cleaning agent to restrict any potential or actual increase of the dynamic wet pressure across the diffusion elements beyond about 25, or 15 or 7 inches of water gauge above a base condition of said elements, and the application of cleaning agent is controlled in accordance with said measuring step to maintain said restriction on dynamic wet pressure.
Three additional embodiments are each applicable to the second aspect and each of its above-described further embodiments. The first of these additional embodiments comprises the step of measuring pressure across and/or flow of gas through one or more diffusion elements which are present in the submerged portion of the network, and which constitute only a portion of said plurality of diffusion elements, for controlling the application of cleaning agent. The second additional embodiment comprises the step of measuring pressure across and flow of gas through one or more diffusion elements which are present in the submerged portion of the network, and which constitute only a portion of said plurality of diffusion elements, for controlling the application of cleaning agent. The third additional embodiment comprises the steps of measuring the hydrostatic pressure of said liquid medium at about the elevation of a diffusion element in a submerged portion of the network, measuring the gas pressure within said gas distribution network near an air flow regulating means supplying gas to said diffusion element through a plenum, and measuring gas pressure downstream of the flow regulating means and within the plenum.
The following is yet another embodiment applicable to the first or second aspect of the invention. In this embodiment, the measured parameter or parameters directly indicates dynamic wet pressure across one or more of said diffusion elements.
In still another embodiment applicable to the first or second aspect of the invention, the diffusers of a given plant are installed as a plurality of groups of diffusers situated in one or more tanks, and the respective groups of diffusers have supply pipes for conveying treating gas from outside their respective tank into a distribution conduit on which said diffusers are mounted, said process comprising the steps of effecting measurement of pressure across and/or flow of gas through at least one submerged diffuser in a group of diffusers served by a given supply pipe, said at least one diffuser representing only a portion of the diffusers of said group, and controlling the application of cleaning agent to said group based on such pressure and/or flow measurement. There are three other embodiments applicable to the one just described. According to the first of these, said pressure measurement includes at least monitoring of pressure at said at least one diffuser, and cleaning gas is applied to said group based on said measurement. According to the second of these, said flow measurement includes at least monitoring of flow through said at least one diffuser, and cleaning gas is applied to said group based on said measurement. According to the third of these, said pressure and/or flow measurement includes monitoring of flow and pressure through said at least one diffuser, and cleaning gas is applied to said group based on said measurement.
Still more embodiments based on the first or second aspect comprise the step of measuring the gas pressure within a plenum supplying treating gas to a diffusion element or the step of measuring the hydrostatic pressure of said liquid at about the elevation of a diffusion element in said submerged portion.
Yet other embodiments, based on the first or second aspect, comprise the step of measuring the gas pressure within said gas distribution network near an air flow regulating means supplying gas to said diffusion element. Two additional options are applicable to the embodiments just described. The first of these comprises the steps of measuring the hydrostatic pressure of said liquid medium at about the elevation of a diffusion element in said submerged portion; and comparing said hydrostatic pressure with said gas pressure within said plenum. The second of these comprises the steps of measuring the gas pressure within said gas distribution network near an air flow regulating means supplying gas to said diffusion element through a plenum; and comparing said gas pressure within said gas distribution network near an air flow regulating means with said gas pressure within said plenum.
The following three embodiments are also based on the first and second aspects. One of these comprises the step of measuring the gas pressure within a plenum supplying treating gas to a diffusion element by providing a manometer located outside said liquid medium and connected to a tube attached to a pressure tap in said plenum. A second comprises the step of measuring the gas pressure within said gas distribution network by providing a manometer located outside said liquid medium and connected to a tube attached to a pressure tap in said gas distribution network near an air flow regulating means supplying gas to said diffusion element through a plenum. The third comprises the step of measuring the hydrostatic pressure of said liquid medium at about the elevation of a diffusion element in said submerged portion by providing a manometer located outside said liquid medium and connected to a tube submerged in said liquid medium having an inlet at substantially the elevation of a discharge surface of the diffusion element.